Bacteriophage T7 is a relatively simple virus that has been well characterized physiologically and genetically. Mutations are now available in 25 of the 30 or so T7 genes, including many of the genes involved in replication and maturation of T7 DNA. Mutant strains of E. coli that do not permit normal replication of T7 DNA are also becoming available. Thus, the T7 - E. coli system is an excellent one in which to study the molecular details of DNA replication. Sites at which T7 DNA is cut by different restriction endonucleases will be mapped. Intermediate forms in replication of wild type and mutant T7 DNA will be identified and isolated by sedimentation or gel electrophoresis. Specific restriction endonucleases will be used to analyze the structure and development of concatameric forms of T7 DNA and to analyze how replication begins during infection by T7 mutants in which the normal origin of replication has been deleted.